As a method for producing a polymer film, namely, a film production method, there are a melt extrusion method and a solution casting method. In the melt extrusion method, a polymer in the form of pellets or powders is heated to be melted. The melted polymer is extruded so as to be formed into a thin film as a polymer film. The polymer film is cooled while being conveyed. In the solution casting method, a dope obtained by dissolving a polymer into a solvent is cast onto a casting support to be a casting film. The casting film is peeled from the casting support before the solvent is completely evaporated from the casting film. The polymer film is dried while being conveyed. As described above, in both of the film production methods, the polymer film is subjected to predetermined processes while being conveyed.
As a means for conveying the polymer film, a suction roller is widely used. The suction roller has plural holes for sucking air on its peripheral surface. The holes suck air such that the polymer film is drawn so as to be in contact with the peripheral surface of the suction roller. The polymer film is conveyed in accordance with the rotation of the suction roller. The suction roller is rotated by a motor in a circumferential direction thereof.
In order to accelerate the production speed, it is necessary to enhance the sucking force of the suction roller such that a conveying speed of the polymer film is accelerated. However, the enhancement of the sucking force causes a phenomenon in which a mark of the hole of the suction roller remains on the polymer film.
For the purpose of preventing such a phenomenon, it is possible to grind the peripheral surface of the suction roller. However, in such a case, it is necessary to halt the operation of a production line, thus resulting in production loss. Therefore, the grinding is not preferable. In view of the above, there is proposed a method for coating a soft material having Rockwell hardness of 98HRR or less onto the surface of the suction roller (for example, see Japanese Patent Laid-open Publication No. 6-179556). Further, in order to prevent the phenomenon in which the mark of the hole of the suction roller remains on the polymer film and prevent scratches and wrinkles due to the polymer film slipping with respect to the suction roller, there is proposed a method in which the sucking force of the suction roller is set within a predetermined range and the difference in tension between the upstream side and the downstream side of the suction roller is set to a predetermined value in accordance with the temperature of the polymer film to be conveyed (for example, see Japanese Patent Laid-open Publication No. 2005-306019).
However, since the suction roller draws the polymer film by sucking air through the holes on its peripheral surface, foreign matters in the air or on the surface of the polymer film are adhered to the holes. The foreign matters may cause scratches on the polymer film. Accordingly, in the methods described in the Japanese Patent Laid-open Publications No. 6-179556 and No. 2005-306019, it is impossible to prevent occurrence of the scratches due to the adhesion of the foreign matters on the polymer film.